This invention relates to an apparatus for use in an electrical power supply circuit to reduce voltage stresses that can initiate the re-strike of a circuit breaker switch used to disconnect a capacitor bank from the electric power supply circuit.
Capacitor banks are regularly used in electrical power supply circuits for various purposes. In one circumstance, the capacitor is used to improve the power factor of the load when that load is mainly or to a high degree an inductive load. The second purpose of such capacitor banks is to maintain the voltage level at a required level when that voltage would otherwise be lowered due to voltage drop in the transmission system.
In many of these cases it is necessary to disconnect the capacitor bank from the power supply circuit in response to changes in the condition of the power supply circuit which are monitored.
As is well known, there is significant danger when switching of capacitive loads for restriking to occur in the circuit breaker. This phenomenon arises predominantly due to the fact that a breaker gap extinguishes when current in the breaker goes through zero. The current in the capacitor bank goes through zero when the voltage on the capacitor bank is at a peak. Therefore, the peak voltage is trapped on the capacitor bank when the gap extinguishes prior to the reversal of the source voltage. Thus, assuming that the capacitor voltage remains constant and the source voltage continues to cycle, it will be clear that there will be a voltage applied across the extinguished gap within one-half cycle which is 2.0 times the AC peak voltage and it will be of unidirectional polarity. This stress can lead to significantly increased likelihood of re-striking of the circuit breaker gap.
If re-strike does occur, the electric charge from the capacitor flows quickly back through the arc across the breaker gap giving the arc a large amount of energy which can result in circuit breaker failure. In particular, when re-strike occurs, the capacitor voltage immediately attempts to equalize with the system voltage. However, the supply circuit has some inductance and the first peak of that high frequency oscillatory transient of capacitor voltage in fact overshoots system voltage by an amount nearly equal to the difference between the voltages immediately prior to re-strike. The current is typically interrupted at its first high frequency current zero and the overshoot voltage is trapped on the capacitor bank. The recovery voltage reaches a greater magnitude than that following the first interruption but the contacts are moved further apart and the build-up of dielectric strength may prevent additional re-strikes and further voltage escalation. Multiple re-strikes can produce, due to sudden voltage changes and high frequency oscillations, other relatively higher voltages elsewhere on the system. The probability of re-strike and voltage escalation increases with the frequency of capacitor switching operations which can in some cases occur several times a day.
Surge arrestors, instrument transformers and fuses in the power supply circuit may therefore experience voltages significantly above normal values.
A further factor which can lead to restrike is that the voltage across the gap is of single polarity, either positive or negative depending upon whether interruption occurs at the positive or at the negative voltage. The uni-directional nature of the field across the gap aids in the development of avalanche mechanism which is a mechanism which can lead to arc formation and thus to re-strike of a breaker gap.
As the introduction of capacitor banks into the power supply circuit is increasing, many of the existing breakers are no longer satisfactory for switching of the capacitor banks because of the voltage stresses normally associated with switching capacitive load. Much of the expense involved in introducing a capacitor bank can often be attributed to replacing of existing breakers with complex high speed breakers which are specifically designed, at increased cost, to reduce the possibility of re-strike. Various techniques are used in the breaker design which will not be discussed here. It suffices to say, however, that the designs are of a significantly increased complexity and accordingly of significantly increased cost.